A valve assembly of this type has hitherto been known which is disclosed in the specification of U.S. Pat. No. 4,137,935. In this known valve assembly, as shown in FIG. 1, a dome-like valve member 104 is rotatably supported within a valve housing 102 attached to an inlet port of a pressure-vessel 101. The valve member 104 opens or closes a circular aperture 103 formed in an upper wall of the valve housing 102. A seal ring 105, directed inwards, is arranged in the peripheral edge of the circular aperture 103 and it engages the outer surface of the valve member 104. Reference numerals 106 and 107 denote a hopper and a drive shaft, respectively.
This known valve assembly is satisfactory for relatively massive powdery granules but unsatisfactory for fine powdery granules. More particularly, when transporting the fine powdery granules under a full-open state of the valve as shown in FIG. 1, the fine powdery granules supplied from the hopper 106 and entering into the interior of the valve housing 102 through the circular aperture 103 come in contact with the substantially entire surface of the seal ring 105 except for a portion thereof covered with the valve member 104. As a result, the seal ring 105 will disadvantageously be abraded and become nondurable for long-term use.